1. Field of the Invention
The present invention relates to molded composite interior and entry doors and a manufacturing process used to produce such doors and other products from a compressed mixture of lignocellulosic or cellulosic particles or fibers and aminoplast, phenolic, or diphenylmethane diisocyanate (MDI) thermo-set resins.
2. Description of Relevant Art
Numerous types of interior and exterior doors and methods to manufacture them exist in the prior art as do a number of molded wood composites and manufacturing methods to produce them.
Prior art such as Simmons' U.S. Pat. No. 2,825,099 issued in March 1958, and others, teach processes for manufacturing entry and interior doors from precisely cut and shaped pieces of wood. This process of manufacturing a door, where solid wood is machined into stiles, rails, and panels that are then glued and assembled into a door, is both costly and time consuming. This traditional manufacturing process is also very wasteful as typically half of the wood used in manufacture is converted into trim waste, chips, shavings, and sawdust by the various machining operations. During manufacture the surfaces of the individual components, and after assembly the surfaces of the finished door, must be sanded to achieve a surface smooth enough to produce an aesthetically acceptable painted finish.
In place of using solid wood, processes such as Amoretti's U.S. Pat. No. 5,771,656 issued on Jun. 30, 1998 and others substitute medium density fiberboard (MDF) to produce interior and entry doors. To produce these doors the MDF sheets are cut and machined into individual components that are then assembled and bonded into a completed door. The completed door is then sanded to remove glue residue from the assembly and to produce a uniformly smooth surface that will produce an aesthetically acceptable paint finish. This process is less wasteful, as MDF does not contain the knots, cracks, and other defects that must be cut out of solid wood, but it still produces waste from the various machining operations used. In addition to assembling cut and machined pieces of MDF, there are some manufacturers that produce solid interior and entry doors by taking a single thick, typically 1.375″ or 1.750″, sheet of MDF or low density fiberboard (LDF) and then use routers to machine designs to simulate stiles, rails, and panels into both faces. The door is then trimmed to size and painted to produce a unit that has the appearance of a solid door. While the doors produced using these methods are solid and durable, the expense and waste associated with the various machining process results in a door that is still costly to produce.
The process found in Leavers U.S. Pat. No. 629,448 issued on Jul. 25, 1899, reduced the cost of mass-producing doors by attaching door skins to a frame to produce a hollow core door with the appearance of a solid door. While the use of lower cost materials increases the overall efficiency of manufacturing doors, the process still requires two door skins be produced in one manufacturing process and then bonded to a dimensioned and assembled frame of wood or other materials with sufficient screw-holding capability for mounting the hinges and lock-set in a second manufacturing process. Once assembled, the hollow core door is trimmed before applying the finish. It should be pointed out that a hollow core door is more subject to damage than a solid door and they do not muffle sounds as effectively as a solid door. To mitigate this deficiency, methods such as found in Lynch, et. al. U.S. Pat. No. 5,766,774 issued on Jun. 16, 1998 and others have filled the void between the two door skins with cardboard, foam, particleboard, and other fillers, which increases the cost of the door.
The production of molded items using thermo-set molding resins blended with wood flour, glass fibers, and other materials exists in the prior art. An early example is the use of phenol-formaldehyde resins blended with wood flour that was marketed in the 1920's under the trade name of Bakelite®. These mixtures produced molded parts using both injection molding and compression molding in closed molds. In the case of compression molding, the blends are often pre-heated to form a bulk molding compound that is then subdivided into individual measured charges or shots that are placed inside the cavity of the mold. The molds are then closed and the charge is subjected to heat and pressure until the mixture has cured. This process produces quality molded items but it requires both a more complex manufacturing process and produces a more costly product as it uses only thermo-set molding resins which are more costly and require higher dosing levels than do thermo-set binding resins. In addition, the molding of products from these blends requires the use of elaborate and expensive closed molds that create a sealed cavity to prevent the material from flowing out of the mold during molding. Davis, U.S. Pat. No. 4,734,236, Mar. 29, 1988 and others have produced molded composites using blends of wood fibers and other materials and thermo-set molding resins without the use of pre-heating, but they still require the use of closed molds.
Molded composites produced using blends of thermo-set binding resins are taught by Vanders, U.S. Pat. No. 7,314,585, Jan. 1, 2008 and others. These composites benefit from using lower cost thermo-set binding resins mixed with wood chips, veneers, particles, and fibers to produce a shaped product of uniform to near uniform thickness. They also benefit from the fact that this material does not flow during molding so that simple and lower cost open molds can be utilized. However the surface finishes of these products are not smooth, hard, and paint-ready and as such are used primarily for structural applications, such as door cores, or in other applications where a paint-ready surface finish is not required. To overcome this deficiency, Whelan, et al., U.S. Pat. No. 6,368,528, Apr. 9, 2002 developed a process where an aqueous solution containing resins and other materials is applied to the surface of a wood and resin mat prior to molding so that a smooth, hard, paint-ready surface is produced. While producing a desirable surface finish, this process adds to the cost and complexity of the manufacturing process.
In view of the foregoing disadvantages and limitations found in the prior art of manufacturing doors, there is an interest in and need for improved processes for manufacturing doors and other similar products.